Looks like a question for Xorg esperts (->xorg mailing list). I guess the answer depends if card(s) is programmable enough to turn off VGA stuff and able to handle remapped IO regions, but also I'm not sure if libpciaccess and kernel stuff is already capable of supporting two video devices at the same time.

I have read the "Release Notes" and unfortunately the problem will only be solved in the 1.5 server. It is said to have been present at least since server 1.3 was released so I will try previous and head versions to see if the problem is either absent or solved there.